1. Technical Field
This disclosure generally relates to a semiconductor device having a MOS (metal oxide semiconductor) transistor which provides a source and a drain formed on a semiconductor substrate by having a predetermined distance between the source and the drain and a gate electrode formed between the source and the drain on the semiconductor substrate via a gate insulation film.
2. Description of the Related Art
Recently, in a semiconductor device, in order to increase the speed of digital operations, a gate electrode of the semiconductor device has been formed by a micro-fabrication technology. In addition, a semiconductor device has been used as an analog device such as a power source device. In particular, when the semiconductor device is used as the analog device, temperature dependency and voltage dependency of the semiconductor device influence circuit characteristics of the analog device, and a technology to compensate the influences by the temperature dependency and the voltage dependency has been important.
A MOS transistor in a semiconductor device provides a source and a drain formed on a semiconductor substrate by having a predetermined distance between the source and the drain and a gate electrode formed between the source and the drain on the semiconductor substrate via a gate insulation film. Ends of the source and the drain overlap or abut on a gate electrode when viewed from above the gate electrode (for example, refer to Patent Documents 1 and 2). Or the source and the drain are disposed to have a predetermined distance from the gate electrode when viewed from above the gate electrode (for example, refer to Patent Document 3).
For example, a driver transistor has been installed in a semiconductor device. The driver transistor has a relatively large channel width for driving a next stage element. As an example using the driver transistor, a charging circuit (device) for a mobile telephone is described.
FIG. 7 is a circuit diagram showing a charging device. In FIG. 7, (a) shows a status before charging a battery and a transistor 37 is turned OFF, and (b) shows a status where the battery is being charged. As shown in FIG. 7, a rechargeable battery 31 is connected to a power source 35 (corresponding to a household wall AC outlet) via a charging switch 33. When the transistor 37 is turned ON, the charging switch 33 connected to the transistor 37 via an electrode pad 23 is turned ON, and as shown in FIG. 7(b), a current A flows into the rechargeable battery 31 from the power source 35.
In FIG. 7, the transistor 37 is the driver transistor. That is, the transistor 37 drives the charging switch 33 which is a next stage element. In addition, when the amount of the current A is large, the period to charge the rechargeable battery 31 is short; therefore, a current B flowing into the transistor 37 is required to be large. The current flowing into the transistor 37 is proportional to the channel width of the transistor 37; therefore, the channel width of the transistor 37 is designed to be large.
Next, the driver transistor is described in detail.
FIG. 8 is a cut-away side view of a conventional driver transistor.
As shown in FIG. 8, a LOCOS (local oxidation of silicon) oxide film 43 for determining a driver transistor forming region is formed on a silicon substrate 41. Sources 45 and drains 47 formed of an N type impurity diffusion layer are formed on the silicon substrate 41 at the driver transistor forming region surrounded by the LOCOS oxide film 43. The sources 45 and the drains 47 are alternately formed by having a distance between the sources 45 and the drains 47.
A gate electrode 51 formed of polysilicon is formed on the silicon substrate 41 between the source 45 and the drain 47 via a gate oxide film 49. That is, the plural gate electrodes 51 are formed between the corresponding sources 45 and drains 47. When the driver transistor is viewed from above, an end of the source 45 and an end of the drain 47 overlap corresponding ends of the gate electrode 51. In FIG. 8, the four gate electrodes 51 are shown. However, in order to make the channel width large, generally, some tens of the gate electrodes 51 are formed in the driver transistor.
A first dielectric interlayer (not shown) is formed on the entire surface of the silicon substrate 41 including regions where the sources 45, the drains 47, and the gate electrodes 51 exist. A metal wiring layer (not shown), a second dielectric interlayer (not shown), a protection film (not shown), and so on are formed on the first dielectric interlayer. The plural sources 45 are electrically connected with each other via contact holes (not shown) and the metal wiring layer. In addition, the plural drains 47 are electrically connected with each other via contact holes (not shown) and the metal wiring layer.
As shown in FIG. 8, in the driver transistor, the sources 45 and the drains 47 are alternately disposed at both sides of the corresponding gate electrodes 51. When the driver transistor is turned ON, currents flow in arrow directions shown in FIG. 8. That is, one source 45 operates for two gate electrodes 51 and one drain 47 operates for two gate electrodes 51. Therefore, a large current can flow within a small area.
[Patent Document 1] Japanese Laid-Open Patent Application No. 2002-261273
[Patent Document 2] Japanese Laid-Open Patent Application No. 2001-185724
[Patent Document 3] Japanese Patent No. 3513411
However, in the conventional driver transistor (MOS transistor) in which the end of the source and the end of the drain overlap the corresponding ends of the gate electrode when viewed from above the gate electrode, when temperature rises, the amount of the drain current is lowered. In particular, in an analog circuit, temperature dependency and voltage dependency of the semiconductor device influence characteristics of the analog circuit; therefore, it is preferable that temperature characteristics in a drain voltage and a drain current of the MOS transistor be adjusted.